The present invention relates to a method for monitoring the state of interference of radio waves by each base station in a mobile radio communication system wherein the service area is divided into a plurality of zones and a plurality of radio channels of different frequencies are reused in respective zones over the entire service area with a minimum of co-channel interference. More particularly, the invention is directed to a method for monitoring the state of interference of radio waves by an autonomous base station which monitors the state of interference of radio waves in the radio channel of the same frequency as that of the radio channel of the base station and, when the state of interference goes down below a prescribed value, switches the radio channel to a different one or goes out of service.
In a mobile radio communication system, as shown in FIG. 1, base stations BS.sub.1, BS.sub.2, . . . , BS.sub.M are each provided in one of a plurality of zones Z.sub.1, Z.sub.2, . . . , Z.sub.M forming a service area SA and, for example, radio channels of frequencies f.sub.1, f.sub.2, . . . , f.sub.6 are assigned to the zones Z.sub.1, Z.sub.2. . . , Z.sub.M, respectively, as shown. In this instance, radio channels of the same frequency f.sub.1, for example, are assigned to zones distant from each other, such as Z.sub.1 and Z.sub.6, with particular attention to the suppression of what is called co-channel interference for efficient reuse of a plurality of radio channels of different frequencies over the entire service area to provide for increased utilization factor of frequency.
In the conventional frequency assignment to respective zones, the distance between radio base stations using the same channel frequencies in the individual zones and their transmitting power are determined through theoretical calculation such that radio waves to each of the zones from the others are sufficiently attenuated to make the interference negligibly small.
In this way, the conventional system reuses the frequencies f.sub.1, f.sub.2, . . . , f.sub.6 of the same set with a view to maximizing the utilization factor of radio waves. With such a conventional system, when an unexpected new propagation path appears or an existing propagation path disappears due to a change in the radio wave propagation environment, radio waves of the same frequency from two radio base stations may sometimes interfere with each other. To measure such interference, it is customary in the prior art to radiate radio waves from each of the two radio base stations one at a time, measure their electric field intensities and make a comprehensive analysis of the measured results, thereby judging the range over which the radio waves from each base station exert an influence. Even after the mobile radio communication system is put into service, interference occurs or disappears due to a change in the radio wave propagation environment, for example, when the service area is extended and base stations are set up in new zones, when the frequency allocation scheme is partly or wholly modified, or when a large building or tower is put up or pulled down. Hence, it is necessary, after putting the mobile communication system into service, to check the state of interference with the radio channel of each of the zones using the same frequency; to perform this for each zone, the transmission of radio waves from its base station needs to be stopped. Furthermore, the interference of radio waves is measured using a dedicated measurement device.
To learn the state of interference of radio waves in one of the zones using the same channel frequency, it is general practice in the prior art to measure the field intensity of radio waves that are caused to radiate one by one from other base stations; hence, much time is needed for the measurement and the judgement of the state of interference. In particular, while the system is in service, the field intensities of the radio waves from the other base stations are measured in a region wherein the radio wave from the base station of that one zone is considered to have a sufficiently high reception level, so that the transmission of the radio wave from the base station of that one zone must be stopped; however, since the transmission of a radio wave cannot frequently be stopped while the system is in service, an appreciable amount of time is required to measure the field intensities of radio waves from other base stations. On this account, the measurement cannot be made rapidly following changes in the radio wave propagation environment, and hence gets far behind, sometimes leaving a region of an undesired state of reception in the zone as it is for a considerably long period of time. Moreover, since the measurement is made on the radio wave radiated from only one of a plurality of base stations using the same frequency that are considered to interfere with each other, the mobile radio communication service is impaired during the measurement.